Porous copper/copper oxide xerogel catalyst

ABSTRACT

An electrocatalytic catalyst is provided. The electrocatalytic catalyst includes a xerogel including copper (I) oxide and copper.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Application Ser. No. 63/144,778, filed on Feb. 2, 2021, the entire contents of which is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure is directed to the electrocatalytic synthesis of liquid fuels from carbon dioxide.

BACKGROUND

The removal of carbon dioxide (CO₂) from the atmosphere, followed by its sequestration is being explored. Most sequestration techniques isolate the CO₂, and then injected it into underground formations for storage. Utilization of the CO₂ for fuel generation would lower the cost of the CO₂ removal, as well as lower the total amount of CO₂ released in the atmosphere. The electrosynthesis of liquid fuels from CO₂ is a promising technology for performing this function.

Copper-based catalysts have a potential to synthesis C₂+ chemicals, providing high energy density fuels that may help in providing a sustainable carbon cycle. Accordingly, as a promising technology for carbon utilization, electrocatalytic CO₂ conversion is becoming a significant area for research.

However, the Faradaic efficiency (FE) for a C₂ product on a flat copper surface is limited to about 20% due to the high-energy barrier of the reaction and the competitive hydrogen evolution reaction (HER). Further, the partial current density (J_(product)) is still too small for commercial usage because of the low surface area and unfavorable reaction mechanics. To overcome this, nano-structuring of copper-based alloy and surface engineering is commonly used to improve reaction intermediate binding energy and local reaction environment control.

Among the various attempts to enhance C₂ product selectivity and productivity of CO₂ conversion electrocatalyst, oxide-derived Cu is of particular interest as it promotes CO binding and the following C—C coupling in the reaction step. Copper catalysts prepared from copper oxides could improve selectivity for C₂ production by residual oxygen or Cu⁺ atoms and under-coordinated surface Cu atom. For instance, Cu₂O formed by electrochemically deposition, O₂ plasma treatment, thermal annealing, and chemical synthesis achieved faradaic efficiency (FE) for C₂H₄ (FE_(C2H4)) of up to about 60%. However, partial current density (J), which correlates with actual production rate, is too small. Further, no reported catalysts have FE and J for ethanol production above 35% and 20 mA/cm² in H-cell reactor.

SUMMARY

An embodiment described in examples herein provides an electrocatalytic catalyst. The electrocatalytic catalyst includes a xerogel formed from copper oxide and copper.

Another embodiment described herein provides a method for making an electrocatalytic catalyst. The method includes dissolving a copper salt in a water solution, adjusting the pH of the water solution to be basic, adding a reducing agent to the water solution, and separating a product powder from the water solution, wherein the product powder is a copper/copper oxide xerogel.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing showing the morphology of a copper/copper oxide xerogel.

FIG. 2 is a transmission electron micrograph image of the copper/copper oxide xerogel.

FIG. 3 is an inverse FFT image of the transmission electron micrograph showing the composition of the xerogel.

FIG. 4 is a plot of the XRD analysis of the copper/copper oxide xerogel.

FIG. 5 is a plot of the X-ray photoelectron spectroscopy (XPS) Auger copper LMM peak of the copper/copper oxide xerogel for a number of different compositions.

FIG. 6 is a scanning electron micrograph image of the copper/copper oxide xerogel.

FIGS. 7A to 7D are cyclic voltammetry scans used for electrochemical surface area measurements of different copper surfaces.

FIGS. 8A to 8F are XPS Auger copper LMM peaks of copper/copper oxide xerogels (samples 1 to 5, as described herein) at different concentrations of copper using a copper oxide xerogel (FIG. 8F) as a reference.

FIG. 9 is a HR-TEM inverse-FFT mapping images of copper oxide xerogel.

FIGS. 10A to 10C are XRD spectra of copper/copper oxide xerogel and copper oxide xerogel.

FIGS. 11A and 11B are plots of the CO₂ electroconversion performance of a planar copper surface catalyst.

FIGS. 12A and 12B are plots of the CO₂ electroconversion performance of a planar copper oxide surface catalyst.

FIGS. 13A and 13B are plots of the CO₂ electroconversion performance of a copper oxide xerogel catalyst.

FIGS. 14A and 14B are plots of the CO₂ electroconversion performance of a copper/copper oxide xerogel catalyst.

FIG. 15 is a plot comparing the Faraday efficiency of each of the catalysts for the production of ethanol at FE_(EtOH) and C₂/C₁ ratio at −1.14 V vs RHE.

FIG. 16 is a plot comparing the current density profile of copper/copper oxide xerogel to the other catalysts.

FIGS. 17A and 17B are plots comparing the electrocatalytic CO₂ conversion performance of the control catalyst with the various Cu/Cu₂O xerogel samples.

FIG. 18 is a plot comparing the xerogel Cu/Cu₂O catalyst (sample 3) with other types of catalysts.

FIG. 19 is a plot of the stability of a copper/copper oxide xerogel.

FIGS. 20A and 20B are transmission electron micrographs of a xerogel catalyst before and after 5 hours of electrolysis.

FIGS. 21A and 21B are HR-TEM inverse-FFT mapping images of the xerogel catalyst before and after 5 hours electrolysis.

FIGS. 22A and 22B are XPS Auger copper LMM spectra of the xerogel catalyst before and after 5 hours of electrolysis.

FIG. 23 is an LSV plot comparing the electrochemical analysis results of four types of copper catalysts.

FIG. 24 is a Tafel plot from the electrochemical analysis of the four types of copper catalysts, including the Cu/Cu₂O xerogel (sample 3).

FIG. 25 is a schematic diagram of a proposed mechanism of electrochemical CO₂ conversion on a copper/copper oxide xerogel.

FIG. 26 shows Arrhenius 2600 of the Cu/Cu₂O xerogel (sample 3) and control samples from LSV.

FIGS. 27A and 27B are a schematic diagram showing the flow of material in the reactor in comparison to scanning electron micrograph of the Cu/Cu₂O xerogel (sample 3) catalyst.

FIGS. 28A and 28B are plots showing the electrocatalytic performance of the copper/copper oxide xerogel in comparison to other copper materials.

FIG. 29 is a process flow diagram of a method for forming a copper/copper oxide xerogel catalyst for electrocatalytic production of ethanol from atmospheric CO₂.

DETAILED DESCRIPTION

Experimental and theoretical reports suggest catalysts with partial oxidation states of Cu, for example, having both Cu⁺ and Cu⁰ regions, may make effective electrocatalysts. From computational studies, CO₂ activation and CO dimerization energy barrier will be significantly lower on the interface between Cu⁺ and Cu⁰ region while impeding the C₁ product pathway. Therefore, catalyst design for maximizing Cu⁺ and Cu⁰ interfaces may be important for efficient and selective C₂ production.

In examples provided herein, a copper/copper oxide (Cu/Cu₂O) xerogel is synthesized by wet-chemistry. As used herein, a xerogel is a solid formed from a gel by drying with unhindered shrinkage. Xerogels generally have high porosity (15-50%) and high surface area (150-900 m2/g), as well as a very small pore size (1-10 nm). To form an electrode for the CO₂ conversion, the synthesized xerogel was drop-casted on carbon paper. The Cu/Cu₂O xerogel exhibited FE_(C2H4) and FE_(EtOH) up to 40%, one of the highest values for EtOH among catalyst tested.

Furthermore, the partial current density of the production of ethanol (J_(EtOH)) reached 31.2 mA/cm2, which is which is higher than any value noted in tests of comparative copper electrocatalysts or in previous research on copper electrocatalysts. When the Cu/Cu₂O xerogel was used in a flow cell reactor as a gas diffusion electrode (GDE), the J_(EtOH) was increased to 72.1 mA/cm2. A high selectivity to C₂ product was provided by a high interface region between Cu⁰ and Cu⁺ interfaces, which facilitated CO₂ activation and C—C dimerization.

It is believed that the morphology of the porous xerogel structure, which has a high surface area and confined spaces, confines the reaction intermediates in close proximity to the active interface regions, contributing to the high productivity. The increase of ethanol productivity over other catalyst may be ascribed to densely located Cu⁰—Cu⁺ interfaces, which facilitate CO₂ activation and C—C dimerization. As these are the reactions, which control the production of C₂₊ chemicals over hydrogen and C₁ chemicals, the production of ethanol is higher than the competing chemicals.

FIG. 1 is a drawing 100 showing the morphology of a copper/copper oxide xerogel. Cu/Cu₂O xerogel was made by a wet chemical synthesis method using a CuCl₂ precursor, NaOH, and controlled amounts of NaBH₄ as a reducing agent to form the Cu⁰. The product was collected by centrifugation with water and ethanol and dried at room temperature, forming the xerogel structure.

FIG. 2 is a transmission electron micrograph image 200 of the copper/copper oxide xerogel. The Cu/Cu₂O xerogel network is composed of densely interconnected nanoparticles with a size under about 50 nm.

FIG. 3 is an inverse FFT image 300 of the transmission electron micrograph showing the composition of the xerogel. In order to identify and visualize the uniform distribution of Cu and Cu₂O on the surface, high-resolution transmission electron microscopy (HR-TEM) with inverse-fast Fourier transformation (inverse-FFT) mapping was conducted. As shown, a nanoparticle in the Cu/Cu₂O xerogel shows a mixed crystal structure of 0.179 nm, 0.209 nm, and 0.245 nm interlayer spacing of the lattice fringes, which corresponds to Cu (200), (111) and Cu₂O (111) domains, respectively. The domains have a size of about 10 nm. The nanoparticles and domains are randomly distributed on the overall surface of the Cu/Cu₂O xerogel. Thus, the inverse FFT image 300 shows a high density of Cu⁰—Cu⁺ interfaces, which are the active sites for the production of C₂ from the conversion of CO₂.

FIG. 4 is a plot 400 of the XRD analysis of the copper/copper oxide xerogel. The X-ray diffraction (XRD) spectrum also confirms crystalline formation of (111) dominant Cu and (111), (200) dominant Cu₂O in Cu/Cu₂O xerogel. This is in agreement with the interlayer spacing and plane observed in the HR-TEM image in FIG. 3.

FIG. 5 is a plot 500 of the X-ray photoelectron spectroscopy (XPS) Auger copper LMM peak of the copper/copper oxide xerogel for a number of different compositions. The compositions labeled as 1 to 5 correspond to samples 1 to 5, herein. The ratio indicated along the y-axis is the ratio between Cu⁺ and Cu. The Cu 2p and Auger Cu LMM peaks in the XPS further verify the surface chemical state, which has mixture of 22.3% Cu⁺ and dominant Cu⁰ species located at 916 eV and 919.9 eV.

FIG. 6 is a scanning electron micrograph image 600 of the copper/copper oxide xerogel. The scanning electron microscopy (SEM) image 600 shows the highly porous, 3-dimensional nanostructure of the Cu/Cu₂O xerogel.

FIGS. 7A to 7D are cyclic voltammetry (CV) scans used for electrochemical surface area (ECSA) measurements of different copper surfaces. FIG. 7A is a CV scan of a planar Cu surface. FIG. 7B is a CV scan of a planar Cu₂O surface. FIG. 7C is a CV scan of a Cu₂O xerogel. FIG. 7D is a CV scan of a Cu/Cu₂O xerogel (sample 3, as described herein). The measurements were performed between −0.4 and −0.25 V versus an Ag/AgCl (KCl saturated) electrode in a 0.1 molar KCl solution between 25 and 300 mV/s.

The corresponding capacitance (CdI) was determined by using cyclic voltammetry (CV) with a changing scan rate. The CdI of the Cu/Cu₂O xerogel was measured as 5.21 mF/cm2, which is 28 times higher than that of planar Cu (0.224 mF/cm2). Accordingly, the high surface area and porous structure will form catalyst with efficient electrocatalytic CO₂ conversion performance.

FIGS. 8A to 8F are XPS Auger copper LMM peaks of copper/copper oxide xerogels (samples 1 to 5, as described herein) at different concentrations of copper using a copper oxide xerogel (FIG. 8F) as a reference. As shown in FIGS. 8A to 8E, the ratio of Cu₂O to Cu can be controlled by changing the amount of reducing agent in the synthesis procedure, as noted with respect to the examples. As seen in these figures, the Cu/Cu₂O xerogels (samples 1 to 5) in FIGS. 8A to 8E show mixed compositions of Cu and Cu₂O with increasing portion of Cu₂O.

FIG. 9 are HR-TEM inverse-FFT mapping images 900 of copper oxide xerogel. With an excess amount of reducing agent, excess amount of Cu₂O will fully covered by Cu₂O over a Cu core. This is shown by the surface exhibiting a strong Cu⁺ peak in the Auger LMM spectrum.

FIGS. 10A to 10C are XRD spectra of copper/copper oxide xerogel and copper oxide xerogel. Mean domain size was also investigated by using the values of the XRD peaks in the Scherrer equation. The results are shown in Table 1. Despite the Cu₂O to Cu ratio changing in Cu/Cu₂O xerogel samples 1 to 5 and Cu₂O xerogel, the mean domain size of dominant Cu₂O (111) and Cu (111) were maintained at about 5 nm and about 8 nm, respectively.

TABLE 1 Mean Domain Size Cu₂O Cu (111) Molar ratio Mean domain size (111) (nm) (nm) of Cu⁺ to Cu Cu₂O xerogel 5.12 8.423 41.6 Cu/Cu₂O sample 5 5.04 8.506 32.5 Cu/Cu₂O sample 4 5.16 8.245 28.3 Cu/Cu₂O sample 3 5.65 8.135 22.3 Cu/Cu₂O sample 2 5.10 8.222 15.3 Cu/Cu₂O sample 1 5.58 8.102 10.4

FIGS. 11A, 11B, 12A, 12B, 13A, 13B, 14A, and 14B show FE to CO₂ conversion products with applied potential between −0.84 and −1.34 V vs RHE for Cu/Cu₂O xerogel (sample 3) and planar Cu, planar Cu₂O, Cu₂O xerogel as a control sample. FIGS. 11A and 11B are plots of the CO₂ electroconversion performance of a planar copper surface catalyst. FIGS. 12A and 12B are plots of the CO₂ electroconversion performance of a planar copper oxide surface catalyst. FIGS. 13A and 13B are plots of the CO₂ electroconversion performance of a copper oxide xerogel catalyst. FIGS. 14A and 14B are plots of the CO₂ electroconversion performance of a copper/copper oxide xerogel catalyst.

The evaluation in FIGS. 11A, 11B, 12A, 12B, 13A, 13B, 14A, and 14B was performed by drop casting the Cu/Cu₂O xerogel (sample 3) with a mixture of Nafion® in methanol solvent. The xerogel structures on the carbon paper electrode maintained the porous structure and the selectivity and productivity toward various products was analyzed under different applied potential with iR correction. An H-cell reactor was used with a CO₂-saturated 0.1 M KCl electrolyte for stable preservation of Cu₂O phase during electrolysis and better suppression of the undesired HER.

The control samples, planar Cu and planar Cu₂O, are shown in FIGS. 11A, 11B, 12A, and 12B, respectively. Compared to the planar Cu, the planar Cu₂O showed lower methane formation and higher C₂ product formation at potentials more negative than −1.04 V vs RHE, which is explained as an effect of oxide-derived copper.

The effect of the xerogel structure on C₂ product selectivity was observed in the Cu₂O xerogel, shown in FIGS. 13A and 13B. The HER and C₁ product selectivity were significantly suppressed over the entire potential range with enhanced C₂ selectivity. The CO₂ reduction overpotential was also lower. These changes may be attributed to the confinement of the intermediates in proximity to the catalytically active sites and the change of local pH environment in the highly porous xerogel structure.

The Cu/Cu₂O xerogel sample (using sample 3 as described with respect to Table 1), provided the results shown in FIGS. 14A and 14B. The Cu/Cu₂O xerogel sample gave the highest C₂ production among the other catalysts, especially for the production of ethanol. The high count of Cu⁰—Cu⁺ interfaces on the highly porous xerogel structure substantially suppressed HER and C₁ production, and facilitated CO₂ activation and C—C dimerization. Accordingly, the highest FE for ethanol and ethylene reached 41.2% and 39.6% at −1.14 V vs RUE.

FIG. 15 is a plot 1500 comparing the Faraday efficiency of each of the catalysts for the production of ethanol at FE_(EtOH) and C₂/C₁ ratio at −1.14 V vs RUE. This comparison illustrates the difference in electrocatalytic CO₂ conversion activity of the samples. The Cu/Cu₂O xerogel (sample 3) achieves a 7.49-fold and 5.5-fold enhancement in FE_(EtOH) over planar Cu and planar Cu₂O, respectively. In addition, the ratio of C₂/C₁ reached about 10, which is a 10.9-fold enhancement over planar Cu.

The xerogel structure significantly improved selectivity to ethanol, which increased FE_(EtOH) from 7.5% (for planar Cu₂O) to 18.5% (Cu₂O xerogel). Furthermore, introduction of greater numbers of Cu⁰—Cu⁺ interfaces dramatically lifted C₂/C₁ ratio and achieved the highest ethanol selectivity of the catalysts.

FIG. 16 is a plot 1600 comparing the current density profile of copper/copper oxide xerogel to the other catalysts. Partial current density reveals actual productivity of an electrocatalyst. The Cu/Cu₂O xerogel (sample 3) shows high productivity, as measured by current density (J_(EtOH)), versus other catalysts. A current density (J_(EtOH)) of 31.2 mA/cm² was achieved with the Cu/Cu₂O xerogel (sample 3) at −1.14 V vs RHE. This value is 45.6 and 38.7 times higher than the current densities of planar Cu and planar Cu₂O, respectively.

FIGS. 17A and 17B are plots comparing the electrocatalytic CO₂ conversion performance of the control catalyst with the various Cu/Cu₂O xerogel samples. The performance of samples 1 to 5 of the Cu/Cu₂O xerogel were measured to determine which ratio of Cu and Cu₂O gave the best results. As sample 3 provided the best results, it was used for other tests described herein, such as those described with respect to 14A and 14B, among others.

FIG. 18 is a plot comparing the xerogel Cu/Cu₂O catalyst (sample 3) with other types of catalysts. The xerogel catalysts described herein provide the highest productivity of electrocatalytic CO₂ conversion to ethanol in H-cell of the currently tested catalyst, as well as other types of copper catalysts from the literature. The high productivity and selectivity to ethanol are believed to be to the large surface area provided by the xerogel, which increases active reaction sites over other types of catalysts.

FIG. 19 is a plot 1900 of the stability of a copper/copper oxide xerogel. The plot 1700 shows that the Cu/Cu₂O xerogel (sample 3) exhibits relatively stable FE and partial current density during ethanol production. The Cu/Cu₂O xerogel (sample 3) retained 80.1% of its FE_(EtOH) with a stable total current density following 5 h electrolysis.

The stability was further explored by imaging the Cu/Cu₂O xerogel (sample 3) catalyst before and after five hours of operation as shown in FIGS. 20A, 20B, 21A, 21B, 22A, and 22B. FIGS. 20A and 20B are transmission electron micrographs of the Cu/Cu₂O xerogel (sample 3) catalyst before and after 5 hours of electrolysis. FIGS. 21A and 21B are HR-TEM inverse-FFT mapping images of the Cu/Cu₂O xerogel (sample 3) catalyst before and after 5 hours electrolysis. FIGS. 22A and 22B are XPS Auger copper LMM spectra of the xerogel catalyst before and after 5 hours of electrolysis. After five hours of electrolysis, the Cu/Cu₂O xerogel (sample 3) maintained a nearly uniform mixture of Cu⁰ and Cu⁺. Some morphological changes were seen due to aggregation and sintering. The stability is likely due to the stabilizing effect of the KCl electrolyte on the Cu₂O.

FIG. 23 is an LSV plot 2300 comparing the electrochemical analysis results of four types of copper catalysts. The electrochemical analysis was performed to explain the high performance of the Cu/Cu₂O xerogel (sample 3). Linear sweep voltammetry (LSV) displays much higher current density with the Cu/Cu₂O xerogel compared to the control samples during the overall potential range. In addition, the onset potential was −0.398 V vs RHE for Cu/Cu₂O xerogel (sample 3), which is less negative than those of other catalysts (−0.488 V, −0.623 V, −0.658 V vs RHE for Cu₂O xerogel, planar Cu₂O and planar Cu, respectively).

FIG. 24 is a Tafel plot 2400 from the electrochemical analysis of the four types of copper catalysts, including the Cu/Cu₂O xerogel (sample 3). The Tafel plot 2400 is based on the relationship of the rate of an electrochemical reaction to the overpotential for the electrochemical reaction. The Tafel plot 2400 for ethanol production was determined to provide kinetic insight from current density for ethanol at various overpotentials. The slopes of the lines in the Tafel plot were 68.9 and 132 mV per decade (dec⁻¹) for Cu/Cu₂O xerogel (sample 3) and planar Cu, respectively, indicating that the overall rate-determining step has changed by introducing the xerogel structure and the Cu⁰—Cu⁺ interfaces. Other studies have indicated that these slopes are similarly observed in CO reduction (CO to C₂ products, 68.9 mV dec⁻¹) and the first electron transfer step in CO₂ reduction (CO₂+e→CO₂.⁻, 132 mV dec⁻¹). This implies that the Cu/Cu₂O xerogel exhibits faster kinetics than other catalysts.

FIG. 25 is a schematic diagram of a proposed mechanism 2500 of the electrochemical conversion of CO₂ to ethanol on the Cu/Cu₂O xerogel. The porous structure of the Cu/Cu₂O xerogel may improve CO₂ reduction selectivity by a high local pH generated via the accumulation of hydroxide ions from CO₂ reduction and HER by-products. The high local pH improves the kinetics of proton adsorption, which may help to suppress HER. In addition, the high concentration of the local intermediates in the confined structure promotes C—C dimerization for C₂ production. Further, the increase in the Cu⁰—Cu⁺ interfaces improves the C—C dimerization kinetics and thermodynamics by a combination of positively and negatively charged CO binding on Cu⁺ and Cu⁰, respectively. Finally, introduction of the increased Cu⁰—Cu⁺ interfaces on xerogel structure exhibits substantial increases in C₂ (ethanol, ethylene) production.

FIG. 26 shows Arrhenius plots 2600 of the Cu/Cu₂O xerogel (sample 3) and control samples from LSV. The Arrhenius plot 2600 can be used to provide the overall activation energy from the absolute value of the slope. The Cu/Cu₂O xerogel (sample 3) shows and absolute value of the slope of 2.61, which is 1.65 times lower than planar Cu. This helps to explain the high intrinsic electrochemical activity.

FIGS. 27A and 27B are a schematic diagram showing the flow of material in the reactor in comparison to scanning electron micrograph of the Cu/Cu₂O xerogel (sample 3) catalyst. To further enhance current density to ethanol, the Cu/Cu₂O xerogel (sample 3) was tested in a flow cell reactor as a gas diffusion electrode (GDE). The Cu/Cu₂O xerogel (sample 3) deposited on hydrophobic polytetrafluoroethylene (PTFE) coated carbon paper was investigated in flow cell reactor composed of one CO₂ gas and two liquid electrolytes (catholyte and anolyte) compartments. At the boundary of catalyst, CO₂ gas and electrolyte, three-phase reaction occurs which can overcome serious limitation of H-cell reactor. High current density could be obtained by removing limitation on gaseous CO₂ solubility on electrolyte. Also, high porosity of Cu/Cu₂O xerogel 3 will gives advantage of exiting the gaseous product rapidly without blocking of bubble while achieving high current density. Cross-sectional SEM image in FIG. 27B shows a GDE which is composed of carbon fiber and microporous layer with the highly porous Cu/Cu₂O xerogel (sample 3) catalyst.

FIGS. 28A and 28B are plots showing the electrocatalytic performance of the copper/copper oxide xerogel in comparison to other copper materials. FIG. 28A shows FE to ethanol using planar Cu and Cu/Cu₂O xerogel (sample 3) in the H-cell reactor and the flow cell reactor. Both the planar Cu and the Cu/Cu₂O xerogel (sample 3) exhibit less negative onset potential for ethanol production in flow cell.

However, selectivity to ethanol reached to about 45% in flow cell with Cu/Cu₂O xerogel (sample 3) under −0.74 V vs RUE. As shown in FIG. 28B, at this potential, the highest J_(EtOH) of 72.1 mA/cm² was achieved, which is a 2.31-fold enhancement over the case of H-cell reactor. This high productivity to ethanol is attributed to the synergetic effects of the efficient Cu/Cu₂O xerogel (sample 3) catalyst and the systematic advantages of a flow cell reactor.

FIG. 29 is a process flow diagram of a method 2900 for forming a copper/copper oxide xerogel catalyst for electrocatalytic production of ethanol from atmospheric CO₂. As used herein the copper/copper oxide xerogel catalyst is an electrocatalytic catalyst that may be used as a solid electrode catalyst, a gas diffusion electrode, or in other forms. The method 2900 begins at block 2902 with the dissolution of copper ions in water. The copper ions can be in the form of copper chloride, or other copper salts, such as copper nitrate, copper sulfate, or copper acetate, among others.

At block 2904, the pH is adjusted to be basic, for example, by the addition of sodium hydroxide. In various embodiments, the pH may be adjusted to be less than about 7.0, less than about 6.5, less than about 6.0, or lower. The adjustment of the pH may trigger the precipitation of copper salts, and the formation of a gel.

At block 2906, a reducing agent is added. In some embodiments, the reducing agent may be sodium borohydride (NaBH₄). Other reducing agents that may be used in embodiments include, LiBH4, NaH, or LiH, among others.

At block 2908, the resulting product is separated from the solution and dried to form a powder. In some embodiments, this may be performed by centrifugation, or filtration, among other techniques. The separated product can be rinsed, for example, with water, alcohols, or both.

At block 2910, a catalyst ink can be formed by suspending the powder in a solvent. In some embodiments, the solvent is an alcohol, such as methanol, ethanol, or isopropanol. In some embodiments, a material is added to the solvent to assist the dissolution, such as an ionomer. In some embodiments, the material is a sulfonated tetrafluoroethylene, such as a Nafion® type available polymer from Chemours of Wilmington, Del., USA.

At block 2912, the catalyst ink can be dropped cast on a substrate to form an electrode. In various examples, the substrate is conductive allowing its use as an electrode. In some examples, the substrate is a carbon film, a carbon rod, a carbon block, and the like. The drop casted catalyst ink is allowed to dry on the substrate. In some embodiments, the drying is performed under an inert atmosphere at ambient conditions. In other embodiments, the drying is performed at an elevated temperature.

The catalyst is not limited to being formed on a conductive substrate, but may be used as a gas diffusion electrode (GDE) deposited on a hydrophilic PTFE paper.

Methods

Synthesis of Cu/Cu₂O Xerogel and Cu₂O Xerogel

15 mL of CuCl₂ (0.1 M) and 15 mL of NaOH (0.2 M) were added into 150 mL of distilled (DI) water with vigorous stirring under nitrogen atmosphere for 30 min. 150 μL of ethanol and controlled amount of NaBH₄ in 25 mL of DI water were quickly added into above solution. Amount of NaBH₄ was 12.5 milligrams (mg), 25 mg, 37.5 mg, 50 mg, 62.5 mg, 70 mg for samples 1 to 5 of the Cu/Cu₂O (copper/copper (I) oxide) xerogel and Cu₂O (copper (I) oxide) xerogel, respectively. After 30 min, the black colored powder was obtained by centrifugation and rinsing with water and ethanol.

Preparation of Planar Cu and Planar Cu₂O

Planar Cu was prepared by electropolishing copper foil in phosphoric acid (85% in water) potentiostatically at 2.1 V vs counter electrode. The planar Cu₂O electrode were fabricated using electrodeposition method at 0.25 V vs ME in 0.5 M CuSO₄ with lactic acid and 5 M NaOH.

Preparation Working Electrode

The catalyst ink was prepared by dispersion of 10 mg of the sample powder with 20 μL Nafion solution (5%) in 1 mL methanol which was ultrasonicated for 1 h. 71 μL of catalyst ink was drop-casted on the 1 cm² carbon paper and dried for 6 h.

Electrochemical Measurements

Electrochemical tests were performed in an H-cell reactor, which is composed of two compartments separated by a proton exchange membrane. 50 mL of 0.1 M KCl electrolyte was injected for each compartment and purged with CO₂ gas for 30 min before CO₂ reduction test. Pt coil and Ag/AgCl (3 M KCl saturated) electrode was used as a counter and reference, respectively. First, working electrode was electrochemically reduced using the cyclic voltammetry (CV) method in the range of −0.5˜−2.0 V vs RHE at the rate of 0.1 V s⁻¹ for 5 cycle. During the constant potential with iR-correction, the gas products were detected by gas chromatograph (Agilent 7890 GC) which is connected to reactor. Liquid products were quantified with 1H nuclear magnetic resonance (NMR Bruker AVANCE III HD). 630 μL of electrolyte after electrolysis mixed with 70 μL of deuterated water (D₂O), 35 μL of 50 mM phenol and 10 mM DMSO for reference.

The potential was converted to RHE using following equation:

E_(RHE) = E_(AgCl) + 0.059  pH + 0.209  V

The FE for products was calculated using the following equation:

F E(%) = nF × V^(*)100j_(Tot)⁻¹

Flow Cell Reactor Electrolysis

Carbon paper with a microporous layer (Sigracet 39 BC, Fuel cell store) was used as a gas diffusion electrode (GDE). The E-beam evaporated Pt on GDE was used as the counter electrode and Ag/AgCl as a working electrode. CO₂ electrolysis was tested in flow cell reactor, which is made of polyetheretherketone (PEEK) and silicone gasket for sealing. Gas flow rate was controlled to 10 sccm via a mass flow controller. CO₂ gas flowing at the backside of cathode GDE was connected to GC and backside of anode GDE was opened to air. The catholyte and anolyte were separated and flow rate was 2 mL min⁻¹. A 1M KCl solution was used as an electrolyte with Nafion proton exchange membrane.

Characterization

The prepared samples were characterized using scanning microscope (FEI Magellan 400), a transmission electron microscope (Tecnai G2 F30), an X-Ray diffractometer (Rigaku SmartLab), and an X-ray photoelectron spectroscope (Axis-Supra). Inverse-fast Fourier transformation (Inverse-FFT) mapping was conducted from FFT data by Gatan Digital Microscopy 3. The double-layer capacitance measurement for ECSA was conducted by CV in a 0.1 M KCl electrolyte. The double-layer capacitance was determined by the value of the slope of the linear fits, and it was considered to be proportional to the ECSA.

Embodiments

An embodiment described in examples herein provides an electrocatalytic catalyst. The electrocatalytic catalyst includes a xerogel including copper (I) oxide and copper.

In an aspect, the xerogel includes a ratio of copper (I) to copper (0) of between 10% and 40% copper (I). In an aspect, the xerogel includes a ratio of copper (I) to copper (0) of 20% copper oxide.

In an aspect, the xerogel is cast on a conductive substrate to form a catalytic electrode. In an aspect, the conductive substrate includes carbon. In an aspect, the electrocatalytic catalyst includes a partial current density for production of ethanol of greater than about 30 mA/cm2.

In an aspect, the xerogel is drop casted on a nonconductive substrate. In an aspect, the xerogel includes a gas diffusion electrode. In an aspect, the electrolytic catalyst includes a partial current density for production of ethanol of greater than about 70 mA/cm2.

In an aspect, the xerogel includes a main domain size for domains of copper of between 8 nm and 8.6 nm. in an aspect, the xerogel includes a main domain size for domains of the copper oxide of between 5 nm and 6 nm.

Another embodiment described herein provides a method for making an electrocatalytic catalyst. The method includes dissolving a copper salt in a water solution, adjusting the pH of the water solution to be basic, adding a reducing agent to the water solution, and separating a product powder from the water solution, wherein the product powder is a xerogel including copper and copper (I) oxide.

In an aspect, the method includes dissolving copper sulfate in the water solution. In an aspect, the method includes adding sodium hydroxide to the water solution. In an aspect, the method includes, after adjusting the pH, stirring the water solution under an inert atmosphere for greater than 30 minutes.

In an aspect, the method includes adding the reducing agent by adding ethanol to the water solution, and adding sodium borohydride to the water solution. In an aspect, an amount of the sodium borohydride is adjusted to control a ratio of copper (I) to copper (0).

In an aspect, the method includes separating the product powder from the water solution by centrifugation. In an aspect, the method includes rinsing the product powder with water and ethanol.

In an aspect, the method includes forming a catalyst ink from the product powder. In an aspect, the method includes forming the catalyst ink by dispersing the product powder in methanol, and ultrasonicating the dispersion for about 1 hour.

In an aspect, the method includes adding an ionomer to the methanol with the product powder. In an aspect, the method includes adding a sulfonated tetrafluoroethylene as the ionomer. In an aspect, the method includes casting the catalyst ink on a substrate. In an aspect, the method includes placing droplets of the catalyst ink on a conductive carbon surface. In an aspect, the method includes placing droplets of the catalyst ink on a polytetrafluoroethylene paper. In an aspect, the method includes drying the catalyst ink on the substrate.

Other implementations are also within the scope of the following claims. 

What is claimed is:
 1. An electrocatalytic catalyst, comprising a xerogel comprising copper (I) oxide and copper.
 2. The electrocatalytic catalyst of claim 1, wherein the xerogel comprises a ratio of copper (I) to copper (0) of between 10% and 40% copper (I).
 3. The electrocatalytic catalyst of claim 1, wherein the xerogel comprises a ratio of copper (I) to copper (0) of 20% copper oxide.
 4. The electrocatalytic catalyst of claim 1, wherein the xerogel is cast on a conductive substrate to form a catalytic electrode.
 5. The electrocatalytic catalyst of claim 4, wherein the conductive substrate comprises carbon.
 6. The electrocatalytic catalyst of claim 1, comprising a partial current density for production of ethanol of greater than about 30 mA/cm2.
 7. The electrocatalytic catalyst of claim 1, wherein the xerogel is drop casted on a nonconductive substrate.
 8. The electrocatalytic catalyst of claim 7, comprising a gas diffusion electrode.
 9. The electrocatalytic catalyst of claim 7, comprising a partial current density for production of ethanol of greater than about 70 mA/cm2.
 10. The electrocatalytic catalyst of claim 1, wherein the xerogel comprises a main domain size for domains of copper of between 8 nm and 8.6 nm.
 11. The electrocatalytic catalyst of claim 1, wherein the xerogel comprises a main domain size for domains of the copper oxide of between 5 nm and 6 nm.
 12. A method for making an electrocatalytic catalyst, comprising: dissolving a copper salt in a water solution; adjusting the pH of the water solution to be basic; adding a reducing agent to the water solution; and separating a product powder from the water solution, wherein the product powder is a xerogel comprising copper and copper (I) oxide.
 13. The method of claim 12, comprising dissolving copper sulfate in the water solution.
 14. The method of claim 12, comprising adding sodium hydroxide to the water solution.
 15. The method of claim 12, comprising, after adjusting the pH, stirring the water solution under an inert atmosphere for greater than 30 minutes.
 16. The method of claim 12, comprising adding the reducing agent by: adding ethanol to the water solution; and adding sodium borohydride to the water solution.
 17. The method of claim 16, wherein an amount of the sodium borohydride is adjusted to control a ratio of copper (I) to copper (0).
 18. The method of claim 12, comprising separating the product powder from the water solution by centrifugation.
 19. The method of claim 12, comprising rinsing the product powder with water and ethanol.
 20. The method of claim 12, comprising forming a catalyst ink from the product powder.
 21. The method of claim 20, comprising forming the catalyst ink by: dispersing the product powder in methanol; and ultrasonicating the dispersion for about 1 hour.
 22. The method of claim 21, comprising adding an ionomer to the methanol with the product powder.
 23. The method of claim 22, comprising adding a sulfonated tetrafluoroethylene as the ionomer.
 24. The method of claim 20, comprising casting the catalyst ink on a substrate.
 25. The method of claim 24, comprising placing droplets of the catalyst ink on a conductive carbon surface.
 26. The method of claim 24, comprising placing droplets of the catalyst ink on a polytetrafluoroethylene paper.
 27. The method of claim 24, comprising drying the catalyst ink on the substrate. 